How to solve the problem of uneven carving bottom in RDC8445S control system?

I used the RDC8445S control system for carving and found that the bottom of the carving was uneven. After careful inspection, I found that this problem occurred in different areas, especially in larger or deeper parts of the carving. I suspect that it is due to insufficient Z-axis accuracy of the engraving machine, or small errors in the stepper motor and transmission system, which prevent stable depth from being maintained during the engraving process. Who can help me solve this problem? Thanks in advance.